Gas-filled optical fiber for wavelength calibration or measurement

ABSTRACT

A gas cell for wavelength calibration or measurement comprises an optical fiber containing a gas having at least one absorption line for providing the wavelength calibration or measurement. The gas is preferably provided in a way that a sufficient part of an optical mode field distribution in the fiber is localized within the gas. The gas may be provided in a hole or an arrangement of holes in or along the fiber.

BACKGROUND OF THE INVENTION

The present invention relates to wavelength calibration.

Currently, reference signals for wavelength calibration of instrumentsand systems used, e.g. in telecommunications, are obtained from opticalabsorption or emission lines of electronic or vibrational states ofmolecules, such as acetylene, HCN, or CO₂, which are contained inconventional glass cells. Details are disclosed e.g. in U.S. Pat. No.6,249,343, U.S. Pat. No. 5,450,193, U.S. Pat. No. 5,521,703, or inhttp://www.boulder.nist.gov/div815/srms.htm.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved wavelengthcalibration. The object is solved by the independent claims. Preferredembodiments are shown by the dependent claims.

According to the present invention, an optical fiber is applied as a gascell for wavelength calibration purposes. The optical fiber preferablycomprises a hole or an arrangement of holes in or along the fiber, inwhich a sufficient part of the optical mode field distribution islocalized. The hole or the arrangement of holes is filled with the gasfor providing absorption lines for the wavelength calibration.

Mode-guiding in the fiber can be achieved preferably in two ways:

-   -   An arrangement of holes acts as an effective medium with lower        refractive index than other regions of the fiber, e.g., the        solid glass core of the fiber. In this case, the mode is usually        guided in the glass of the fiber core, and only a small portion        of the field distribution is localized in the holes. However, an        arrangement of regions (or “shells”) with different hole        densities can also be applied which mimics a profile of the        effective index of refraction analogous to that in a        conventional optical fiber. In this case, the fraction of the        mode density localized in the holes will be larger.    -   An arrangement of holes acts as a photonic crystal which has        very high reflectivity for modes guided in the region surrounded        by the photonic crystal region. This region can be a very large        diameter “hollow core” which then guides most of the mode        intensity.

According to the invention, the holes in such fiber are filled with adefined gas or gas compound used as wavelength reference standard. Theuse of such fiber gas cells thus allows to enormously increase theinteraction length of the light with the gas molecules compared to onlya few cm in conventional gas cells. Therefore gases with rather lowabsorption, such as CO₂, can be used. This is especially useful in thetelecommunications L band.

Further, the inventive fiber gas cells can be provided more compact,more flexible and better suited to fiber-optic instruments than thebulky cuvette-type conventional cells used today. Problems ofpig-tailing and free-space connections across free path lengths ofseveral cm can be significantly reduced.

Additionally, the volume of toxic gases, e.g. HCN, required for someapplications can be significantly smaller. This has benefits formanufacturers, operators, and environment. Finally, fiber gas cells canbe provided cheaper than conventional ones. Only a few meters of fiberare needed at most.

In a preferred embodiment for making the inventive fiber gas cells,air-filled hollow cores of “normal” photonic crystal fibers are filledwith a desired gas or gas mixture. This can be achieved e.g. by pumpingon one side and attaching a gas reservoir on the other side of thefiber. End pieces consisting of flat glass, microlenses as well as otheroptical, source or detection elements could be attached, for example bygluing or arc welding methods.

Alternatively, small pieces of frozen gas crystals or small amounts ofliquid gas can be inserted in the evacuated fiber that is then sealed.The fiber fills with gas as the crystals or the liquid evaporate.

Since gas filling of holes with small diameters might suffer from thelarge resistance of the very narrow channels, the whole fiber growthprocess is preferably performed in another embodiment in an environment(e.g. under pressure) of the desired gas or gas mixture.

In a preferred embodiment, the optical fiber is provided in accordancewith a hollow-core fiber as disclosed by J. C. Knight et al., OpticsLetters 21, 1547 (1996), a “holey” fiber as disclosed by M. Ibanescu etal., Science 289, 415 (2000), or a photonic crystal fiber as disclosedby J. Broeng et al., Danish Opt. Soc. News, p. 22, June 2000 or J.Broeng et al., J. Opt. A: Pure Appl. Opt. 1, 477 (1999) or J. Broeng etal., Science 285, 1537 (1999.

Other applicable fiber structures are disclosed e.g. in WO-A-0022466,WO-A-9964903, WO-A-9964904, U.S. Pat. No. 6,301,420, WO-A-0142831,WO-A-0065386, or WO-A-0016141.

For providing a wavelength reference measurement, the inventive fiberfilled with gas having known absorption wavelengths is preferablycoupled to a wavelength source providing the stimulus for the gas-filledfiber. A wavelength response signal of the gas-filled fiber in responseto the applied stimulus is detected and analyzed. Comparing the detectedwavelength response signal with the known absorption wavelengths thenallows calibrating the provided wavelength analysis using the knownabsorption wavelengths. Calibration schemes and setups as disclosed e.g.in the aforementioned U.S. Pat. No. 6,249,343, U.S. Pat. No. 5,450,193,U.S. Pat. No. 5,521,703, or inhttp://www.boulder.nist.gov/div815/srms.htm, as well as other knownwavelength measurement, control and calibration techniques, can beapplied accordingly.

Further preferred embodiments are:

-   -   The individual holes of the fiber gas cell are not all uniformly        filled with the same gas used for wavelength calibration. Other        possibilities include: (1) Some of the holes are filled with the        reference gas and some holes are under vacuum (“empty”); (2)        some of the holes are filled with the reference gas and others        are filled with another gas, e.g. air. The gas cell, however,        should be provided in a way that interaction of the light with        the reference gas is strong enough to allow for wavelength        measurement.    -   Different holes of the fiber gas cell are filled with different        reference gases, e.g., C₂H₂ and CO₂ in one and the same fiber.        This allows the simultaneous measurement of reference        wavelengths in different spectral regions, according to the        gases used, in a single fiber gas cell.    -   At least two fiber gas cells having a certain length and being        filled with different reference gases, e.g., C₂H₂ and CO₂, are        spliced together, thereby forming a new fiber gas cell having a        greater length. This arrangement allows the simultaneous        measurement of reference wavelengths in different spectral        regions, according to the gases used, in a single fiber gas        cell.    -   A fiber gas cell having at least one end piece consisting of a        lens or another means to improve the coupling of this fiber gas        cell to other fiber-optical components and systems. The at least        one end is mechanically coupled or fusion spliced to the fiber        gas cell.    -   Fiber gas cell in combination with an optical system, such as        but not limited to a source or receiver of optical power, to        perform wavelength reference measurements.    -   An integrated system of fiber gas cell with light source and/or        detector mounted directly onto the fiber ends for easy        incoupling and/or detection of optical power.    -   Fiber gas cell using the broadband light from the spontaneous        emission (SSE) of a laser as input illumination. Such a unit        may, e.g., replace the combination of light-emitting diode (LED)        and conventional gas cell used for wavelength calibration of an        optical spectrum analyzer (OSA), since the SSE could be obtained        from a tunable laser that is oftentimes used together with an        OSA. In an OSA using heterodyne technology, the SSE could also        be obtained from a built-in laser source.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the attendant advantages of the presentinvention will be readily appreciated and become better understood byreference to the following detailed description when considering inconnection with the accompanied drawings. Features that aresubstantially or functionally equal or similar will be referred to withthe same reference sign(s).

FIG. 1 shows a setup for providing a wavelength reference measurementaccording to the present invention.

FIG. 2 illustrates, in cross sectional view, in principle an embodimentof the fiber 10 according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a fiber 10 filled with a gas having known absorptionwavelengths is coupled to a wavelength source 20 providing a stimulussignal for the gas-filled fiber 10. A wavelength response signal of thegas-filled fiber 10 in response to the applied stimulus is detected by adetector 30 and analyzed by an analyzing unit 40. The analyzing unit 40compares the detected wavelength response signal with the expectedabsorption wavelengths known for the gas in the fiber 10. Differencesbetween actually measured absorption wavelengths and the expectedabsorption wavelengths then allow calibrating the provided wavelengthanalysis of the analyzing unit 40.

FIG. 2 illustrates in principle, in cross-sectional view, an applicableembodiment of the fiber 10, as known from: J. Broeng et al., Danish Opt.Soc. News, p. 22, June 22. The regular pattern of circles 100 denotesholes filled with gas. The large cross-sectional area 110 in the centerof the figure, having exemplary hexagonal symmetry, represents thehollow core of the fiber 10 and is also filled with gas. The almostcircular gray-scale image denotes the field distribution of thefundamental guided mode of the fiber that occurs mainly in thegas-filled region.

1. An optical fiber containing a gas providing at least one absorptionline for providing a wavelength calibration or measurement.
 2. Theoptical fiber of claim 1, wherein the gas is provided in a way that asufficient part of an optical mode field distribution in the fiber islocalized within the gas.
 3. The optical fiber of claim 1, wherein thegas is provided in a hole or an arrangement of holes in or along thefiber, in which a sufficient part of the optical mode field distributionis localized.
 4. The optical fiber according to claim 1, wherein anarrangement of holes in the fiber acts as an effective medium with lowerrefractive index than other regions of the fiber.
 5. The optical fiberaccording to claim 1, wherein an arrangement of regions or shells withdifferent hole densities provides a profile of the effective index ofrefraction analogous to that in a conventional optical fiber.
 6. Theoptical fiber according to claim 1, wherein an arrangement of holes actsas a photonic crystal which has high reflectivity for modes guided inthe region surrounded by the photonic crystal region.
 7. The opticalfiber according to claim 1, wherein some holes in the fiber are filledwith the reference gas and some holes are substantially under vacuum orfilled with a different gas.
 8. The optical fiber according to claim 1,wherein different holes of the fiber are filled with different referencegases.
 9. The optical fiber according to claim 1, further comprising atleast one end piece, preferably a lens, for better coupling to otherfiber-optical components or systems.
 10. A gas cell for wavelengthcalibration or measurement comprising an optical fiber containing a gasproviding at least one absorption line for providing a wavelengthcalibration or measurement.
 11. A gas cell for wavelength calibration ormeasurement comprising a plurality of optical fibers containing a gasproviding at least one absorption line for providing a wavelengthcalibration or measurement, each having a certain length and beingfilled with a respective reference gas, wherein the plurality of opticalfibers are spliced or otherwise coupled together.
 12. An optical systemfor perform a wavelength reference measurement, comprising: an opticalfiber or a gas cell for wavelength calibration or measurement comprisingan optical fiber containing a gas providing at least one absorption linefor providing a wavelength calibration or measurement, adapted forreceiving an optical stimulus signal, a receiver adapted for receiving aresponse signal of the optical fiber to the applied optical stimulussignal, and a processing unit adapted for determining in the responsesignal one or more wavelengths absorbed by the optical fiber or the gascell.
 13. The optical system of claim 12, wherein the processing unit isadapted to comparing the one or more determined absorption wavelengthswith known one or more absorption wavelengths for providing a wavelengthcalibration.
 14. A method for making an optical fiber or a gas cellcontaining a gas providing at least one absorption line for providing awavelength calibration or measurement, comprising the step of: fillingat least one hole or air-filled hollow core of a photonic crystal fiberwith a desired gas or gas mixture.
 15. The method of claim 14, furthercomprising the steps of: pumping on one side of the fiber, and attachinga gas or liquid gas reservoir on the other side of the fiber.
 16. Themethod of claim 14, further comprising the steps of: inserting pieces offrozen gas crystals or liquid gas in the evacuated fiber, and sealingthe fiber.
 17. The method of claim 14, being performed in an environmentof the desired gas or gas mixture.